CVE-2016-8743 in macOS
Summary
by MITRE
Apache HTTP Server, in all releases prior to 2.2.32 and 2.4.25, was liberal in the whitespace accepted from requests and sent in response lines and headers. Accepting these different behaviors represented a security concern when httpd participates in any chain of proxies or interacts with back-end application servers, either through mod_proxy or using conventional CGI mechanisms, and may result in request smuggling, response splitting and cache pollution.
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Analysis
by VulDB Data Team • 01/21/2021
The Apache HTTP Server vulnerability CVE-2016-8743 represents a critical security flaw in the server's handling of whitespace characters within HTTP requests and responses. This vulnerability affects all Apache HTTP Server versions prior to 2.2.32 and 2.4.25, where the server exhibited overly permissive behavior regarding whitespace validation in HTTP protocol elements. The issue stems from the server's failure to properly enforce strict HTTP protocol compliance when processing whitespace characters in request lines, headers, and response data.
The technical flaw manifests through the server's liberal interpretation of whitespace characters including spaces, tabs, and other control characters that should be strictly validated according to HTTP specifications. When Apache HTTP Server encounters these non-compliant whitespace sequences, it processes them in a manner that deviates from standard HTTP protocol requirements. This behavior creates a security risk because the server accepts and processes whitespace variations that other HTTP components in the chain might not handle consistently. The vulnerability specifically impacts the server's interaction with proxy chains and backend application servers, where inconsistent whitespace handling between components can lead to serious security implications.
The operational impact of CVE-2016-8743 extends beyond simple protocol violations to encompass significant security risks including request smuggling, response splitting, and cache pollution scenarios. Request smuggling occurs when an attacker manipulates whitespace variations to inject malicious requests that may bypass security controls or be interpreted differently by intermediate proxies. Response splitting vulnerabilities arise when the server's inconsistent whitespace handling allows attackers to craft responses that contain multiple distinct HTTP responses, potentially enabling cache poisoning attacks. Cache pollution represents another serious consequence where improperly handled whitespace causes cached content to be incorrectly stored or retrieved, leading to potential information disclosure or service disruption.
This vulnerability aligns with CWE-117, which addresses improper output neutralization for logs, and relates to ATT&CK technique T1190, which covers exploitation of remote services through protocol manipulation. Organizations using Apache HTTP Server in environments with proxy chains or backend integration face particular risk, as the vulnerability's impact multiplies when multiple HTTP components in the chain exhibit inconsistent whitespace handling behaviors. The security implications are particularly severe in enterprise environments where Apache serves as a reverse proxy or load balancer component in complex infrastructure deployments.
Mitigation strategies for CVE-2016-8743 require immediate deployment of patched Apache HTTP Server versions 2.2.32 and 2.4.25 or later. Organizations should also implement comprehensive network monitoring to detect unusual whitespace patterns in HTTP traffic and consider deploying web application firewalls with enhanced HTTP protocol validation capabilities. Additionally, security teams should conduct thorough assessments of their proxy configurations and backend server integrations to identify potential exploitation vectors. Regular security audits of HTTP protocol implementations and enforcement of strict whitespace validation policies across all network components will help prevent similar vulnerabilities from emerging in the future.